Sigma Level

Also known as the Z score, it measures the voice of the process (VOP) in relation to the voice of the customer (VOC). In a sense, it measures the "severity of pain" in the process not meeting the customer's requirements. Technically, it measures the number of standard deviations a data point (like the upper specification limit or USL) is from the mean. The calculation is ((X - μ) / σ) where X is an observation like the USL.
Related StatStuff Video: 
   -Section 6-Analyze Phase, Lesson #5 - Process Capability: Step 4 (Normal Distributions)